Inductive loops are the most widely deployed traffic sensors and serve as the standard form of traffic detection in the existing road infrastructure. They are a major data source for Advanced Traveler Information Systems (ATIS) and Advanced Traffic Management Systems (ATMS). Most actuated intersection signal control systems, freeway monitoring systems, and traffic event data collection systems use loop detectors for traffic detection. Loop detectors may be deployed as single loops to measure traffic volume and lane occupancy or dual loops to collect speed and vehicle length data in addition to the single-loop measurements.
FIG. 1A (Prior Art) shows the components of an exemplary inductive loop detection system 10. An inductive loop detector contains two parts, including a field component 12 and a detector electronics unit (DEU) 14. The field component includes one or more turns of insulated loop wire 16 wound in a shallow slot sawed into the pavement, a pull box 18 at curbside, and a lead-in cable 20 that connects the pull box and the DEU. The DEU is housed in a controller cabinet 22.
Each loop detector is a tuned electrical circuit of which loop wire 16 is the inductive element. When a vehicle 24 drives over the loop wire in a roadway 26, eddy currents are induced around the peripheral metal of the vehicle. Although the iron mass of the vehicle's engine, transmission, or differential will increase the loop inductance due to the ferromagnetic effect, the decrease in inductance from the eddy currents more than offsets the increase from the ferrous mass, and the net effect of the vehicle's presence is an overall reduction in loop inductance. Therefore, when a vehicle is on top of a loop detector, it decreases the inductance of the loop. This decrease in inductance then triggers the DEU's output relay or solid state circuit which, in turn, switches the output voltage to the controller to a low level (close to 0 v), signifying that a vehicle's presence has been detected.
Typically, a controller scans loop detector outputs 60 times a second or at 60 Hz. To assure the effectiveness of actuated intersection signal control systems, freeway monitoring systems, and traffic event data collection systems, hardware and software of the systems should be tested before deployment. However, research laboratories, where these tests are typically conducted, generally do not have the field component of a loop detector system and, therefore, may not have live loop inputs for the tests. Although controller cabinets have some built-in test features, system tests enabled by these features tend to be relatively simple and insufficient to provide required data.
The Controller Interface Device (CID) developed by the National Institutes of Advanced Transportation Technology (NIATT) provides a data exchange interface between computers running simulation software and controllers, so that signal timing plans can be tested with controller hardware in a laboratory before being deployed in the field. The CIDs have enabled hardware-in-the-loop simulations (HITLS) and made traffic simulation results more reliable. However, CIDs are not designed for simulating loop event data for in-laboratory research and education. The frequency of providing loop event data from a CID depends on the microscopic simulation software that drives it. Typically, a microscopic simulation software package has a deterministic frequency of recalculating the position of each vehicle between 1 and 10 Hz, which limits the time resolution of loop event data to the range from about 1/10 to about 1 second. This resolution is much lower than the 1/60 second resolution level for most loop detection systems. Additionally, using a CID for loop event data simulation requires purchasing both simulation software and a CID, which can easily cost around $3,000 or more. Therefore, a loop detector simulator that can generate precise real-time live loop inputs to controllers and traffic event data collection systems at an affordable cost would be desirable for in-laboratory traffic research and training, as well as for other functions.
Testing of traffic control algorithms on an operating controller at an intersection is typically a labor-intensive work under considerable pressure, because any failure may cause significant interruptions to the traffic stream and induce accidents. Therefore, it would be desirable to facilitate in-laboratory testing of controllers and algorithms running on them before onsite deployment of these components and software.